DESCRIPTION: (Investigator's abstract) Although red blood cell (RBC) transfusion is an essential component in the management of acute complications of sickle cell disease (SCD), and the recent "STOP" study has demonstrated that chronic blood transfusion can prevent stroke in high-risk SCD children, transfusion in SCD has associated problems: 1) high alloimmunization rates (up to 30 percent) and iron accumulation; 2) limitation of post-transfusion hematocrit to less than 35 percent to avoid blood hyperviscosity which may precipitate a vaso-occlusive event. However, we have recently developed a technique which has the potential to mitigate these problems: covalent bonding of a thin coating of polyethylene glycol (PEG) or related polymers to the RBC surface. Results to date indicate that consequent to coating, the RBC surface is inaccessible to antibodies (i.e., the RBC blood group antigens are "masked") and RBC interactions, such as RBC aggregation, are minimized; the latter effect results in greatly reduced low-shear blood viscosity even when the hematocrit of SS blood is increased with coated RBC. The ultimate objective of this Research Program is the development of safe and effective RBC polymer coating methods which achieve antigen masking and viscosity reduction and which offer therapeutic benefits for sickle cell disease subjects. Specific aims include: 1) optimizing polymer coating techniques via evaluating linear, branched, star and dendrimer PEG molecules and various bonding chemistries and crosslinking strategies; 2) evaluation of the functional status of polymer-coated RBC in terms of RBC morphology, rheological behavior (i.e., deformability), membrane transport and oxygen binding, identification of membrane proteins affected (e.g., C-14 labeled PEGs), the storage ability of coated-cells, and in vivo survival in mice and rabbit systems; 3) evaluation of polymer-coated RBC as therapeutic agents in SCD via in vitro rheologic studies of M and SS RBC mixtures at various hematocrits and oxygen tensions, and via in vivo flow studies using rat mesocecum and cat skeletal muscle preparations; 4) evaluation of polymer coating as a means to prevent alloimmunization and/or to protect transfused RBC in alloimmunized subjects by utilizing both in vitro (e.g., antibody/complement binding, complement lysis, monocyte monolayer assay) and in vivo approaches (e.g., alloimmunized rabbit model, xenotransfusions) methods. An interactive approach to these aims is proposed; their successful achievement should yield important new data and improved health care in SCD.